Injection tip for casting machine, casting machine and method using such a tip

ABSTRACT

Disclosed is an injection tip for injecting liquid metal under pressure, suitable for being inserted between a liquid metal supply conduit and an injection opening of a mould, the injection tip having a tubular in shape, made from electrically insulating refractory material and in that it includes a channel suitable for the flow of liquid metal between a first end referred to as the upstream end, suitable for being connected to the conduit, and a second end, referred to as the downstream end, suitable for being connected to the mould, and an electromagnetic coil, positioned between the ends, the axis of which merges with at least a portion of the axis of the channel. Also disclosed is a casting machine using such a tip and a method for casting metal using the machine.

The invention relates to an injection tip with a magnetic pinch for a machine for casting metal under pressure, and more particularly to such a tip which can be used for the metal moulding under pressure of metal pieces. The invention also relates to a casting machine and method using this tip.

Machines for casting metal under pressure are generally used for metal moulding under pressure of metal pieces made of aluminium, zinc, magnesium, etc. and alloys thereof. This moulding method permits high production rates and a high level of precision in the pieces obtained. One of the most important parameters of the method is the pressure at which the molten metal in the liquid state is injected into the mould, particularly at the end of injection. This pressure is from 5 to 30 MPa for machines named hot chamber machines and from 25 to 200 MPa, or even 350 MPa for machines named cold chamber machines.

In the present document and for the purposes of simplification the term “liquid metal” will be used to designate a molten metal in the liquid state, without prejudging the nature and/or melting temperature of said metal.

Most of these casting machines use a plunger comprising a piston which pushes the liquid metal in a cylinder, the end of which, opposite to the piston, is connected to the injection opening of a mould. However, these machines have various disadvantages: the piston rapidly becomes worn, in particular during injection of aluminium alloys which dissolve iron at high temperatures, the adjustment of the quantity of material injected depends on the usable volume of the chamber of the plunger which has to be changed in the event of a change in the volume of the piece to be moulded, the energy to be supplied in order to maintain the temperature of the molten metal is high owing to the dissipation of heat through the cylinder and the piston, etc. From e.g. WO02/30596 a machine for injecting under pressure is known, comprising a tubular injection tip comprising a resistor wound on the major part of this tip in order to maintain the temperature of the molten metal. JP2005-28429 also describes a similar injection machine comprising a heating sleeve in which metal bars are inserted and melted by a resistive coil surrounding the sleeve. In order to terminate the supply of liquid metal to the mould, a stopper fixedly attached to the mould can be actuated to close the supply channel. However, in these examples, the injection pressure depends on the general supply pressure of the machine.

For example, FR 2 668 967 therefore proposed machines in which the supply of liquid metal is effected by an electromagnetic pump. However, the performance of these machines in terms of injection pressure, in particular at the end of injection, is relatively poor.

There remains, therefore, a requirement for casting machines which are simpler to operate and maintain, permitting simple and wide-ranging adjustments while retaining high levels of performance.

The invention thus relates to the provision of an injection tip suitable for association with a casting machine, which permits easy adjustment of the quantity of metal injected and high pressure at the end of injection.

The invention also relates to a casting machine comprising such a tip, particularly suitable for permitting high levels of performance to be achieved.

The invention further relates to a method for casting metal, carried out by a casting machine fitted with an injection tip according to the invention, the method being particularly suitable to metal moulding under pressure.

For this purpose, the invention relates to a tip for injecting metal under pressure, being tubular in shape, made from electrically insulating refractory material, suitable for being inserted between a conduit for supplying liquid metal and an injection opening of a mould, comprising an injection channel suitable for the flow of liquid metal between a first end, named upstream end, suitable for being connected to said conduit and a second end, named downstream end, suitable for being connected to the mould, characterised in that said tip comprises an electromagnetic coil placed between said ends, having an axis merged with at least one axis portion of the channel, suitable for being supplied with a current pulse from a high-voltage generator and generating an electromagnetic pinch in the injection channel. By virtue of this tip and in particular of the channel in which the liquid metal flows it is possible to fill the mould placed at the downstream end of the tip. Furthermore, since the material of the tip is refractory, e.g. of ceramic, the heat of the metal flowing in the channel has little degrading effect thereon. Furthermore, by virtue of the electromagnetic coil which surrounds this conduit, when a high-amperage electric current pulse passes through it, the liquid metal flowing in the channel is subjected on the one hand to a magnetic field oriented axially with respect to the coil and to the channel and on the other hand to an induced current opposite to the direction of the current in the coil. The combination of these two phenomena generates Lorentz forces orientated radially with respect to the conduit, in the direction of the axis thereof. These forces cause a violent pinching of the liquid metal in the channel, projecting the metal to both sides of the pinch zone. The projection of the liquid metal in the direction of the injection opening of the mould thus permits the generation of an over-pressure in the metal, which is particularly useful for the quality of finish of the pieces moulded in this manner.

Advantageously and in accordance with the invention, the coil is suitable for being connected to the terminals of a current pulse generator. Such a generator, e.g. a Marx generator, is capable—using one or more spark gaps discharging a bank of capacitors—of developing an electric current of the order of several tens of thousands of amps at a voltage of several thousands of volts in a very short time, of the order of some milliseconds.

Advantageously and in accordance with the invention, the channel has a diameter comprising a narrowing between the upstream end and the coil. By virtue of this narrowing which forms a converging nozzle, the velocity of the liquid metal leaving the conduit for supplying the metal is increased at right angles to the electromagnetic coil owing to the narrowing of the cross-section of the channel. Consequently, when a current is injected into the coil, the pinch produced in the channel, at right angles to the coil, in the narrowed part of the channel, projects the liquid metal at a velocity which is added to the initial velocity of flow of the metal in the direction of the injection opening of the mould, whereas in the opposite direction, this velocity is decreased by the effect of the narrowing which behaves in this sense as a diverging nozzle and is deducted from the initial velocity of flow of the liquid metal. For this reason, the reaction to the pinch in the direction of the conduit for supplying the liquid metal is greatly attenuated.

Advantageously and in accordance with the invention, the coil is embedded in the material of the tip. In order to minimise energy losses, the electromagnetic coil is placed closest to the flow stream of the liquid metal by embedding it in the refractory material forming the tip. Furthermore, being thus held on all sides, the electromagnetic coil is less subjected to the reaction forces acting on its turn(s) upon pinching. It remains possible nevertheless, particularly when the conditions for injection of the metal (temperature, pressure . . . ) permit, to use a tip comprising a thinner wall and thus to place the electromagnetic coil around and on the outside of the nozzle of the tip. In this variation, the holding of the coil is facilitated.

Advantageously and in accordance with the invention, the coil has multiple turns. Alternatively and in accordance with the invention the coil has a single turn. The electromagnetic coil is preferentially produced so as to present a plurality of turns extending from the narrow end of the narrowing of the channel in the direction of its downstream end connected to the injection opening of the mould in order to increase the width of the pinch zone of the liquid metal in the tip and thus the volume of metal propelled towards the mould. Alternatively, the coil can be a single-turn coil which makes it possible to achieve, for a single output stage of the current pulse generator, higher frequencies (or shorter pulses), thus increasing the momentary power of the pulse, a better transfer of the energy and extended service life.

The invention also relates to a casting machine comprising a liquid metal reservoir, a conduit for the supply of liquid metal connected to said reservoir and fitted with an electromagnetic pump suitable for causing the liquid metal to flow in the conduit in the direction of a mould, characterised in that the machine comprises a tip having at least one of the above-mentioned features, between said conduit and an injection opening of said mould. Such a tip renders the casting machine more reliable and easier to maintain. In fact, it is within the tip that the greatest variations in pressure and thus wear phenomena take place. Consequently, a detachable tip renders possible reduced down times during maintenance.

Advantageously and in accordance with the invention, the electromagnetic pump comprises a plurality of induction coils coaxial to the conduit and suitable for induction heating of the metal flowing in the conduit. By installing helical induction coils regularly distributed over the length of the conduit for supplying liquid metal from the reservoir to the tip it is possible to maintain the temperature of the metal and to avoid the formation of lumps.

Advantageously and in accordance with the invention, the induction coils are supplied with polyphase current so as to generate a moving magnetic field and to drive the liquid metal in the direction of the tip. The induction coils supplied with alternating currents for induction heating can form an induction magnetohydrodynamic accelerator when they are supplied with currents having phase difference from one coil to the other. Consequently, apart from the heating of the liquid metal in the supply conduit, the coils make it possible to generate an axial magnetic field in the conduit and to communicate a movement of the liquid metal in the direction of the tip.

Advantageously and in accordance with the invention, the machine comprises a device for cooling the induction coils which is interposed between said coils and the conduit. In order to limit the heating of the induction coils these coils are insulated from the conduit by an air or water cooling device, e.g. for cooling using water, by a circulation of cooling liquid within a copper tube serving as a winding.

Furthermore, the invention relates to a method for casting liquid metal under pressure in a metal mould, wherein:

-   -   a casting machine is used which comprises a liquid metal         reservoir, a conduit connected to said reservoir and fitted with         an electromagnetic pump suitable for causing the liquid metal to         flow in the conduit in the direction of a mould,     -   the electromagnetic pump is supplied with a polyphase current so         as to displace the liquid metal from the reservoir to the         injection opening of the mould,         characterised in that the casting machine comprises a tip         comprising an electromagnetic coil surrounding an injection         channel between said conduit and an injection opening of said         mould and in that, at the end of injection, the electromagnetic         coil of the tip is supplied with an electric current pulse from         a high-voltage generator in order to generate an electromagnetic         pinch in the injection channel and to propel the liquid metal         under pressure towards the injection opening of the mould.

By causing a current pulse to flow in the electromagnetic coil of the tip at the end of injection, a radial electromagnetic force is generated which acts upon the liquid metal flowing in the injection channel. This pinch force forces back the metal from both sides of the cross-section of the channel around which the electromagnetic coil is installed. The metal is thus forced back towards the injection opening of the mould at a velocity which is added to the flow velocity of the metal in the channel under the effect of the electromagnetic pump and generates an over-pressure within the cavity of the mould, thus improving the filling thereof. On the other side of the cross-section in which the pinching force develops, the metal is forced back in the direction of the supply conduit at a velocity which is deducted from the flow velocity of the metal. The resulting flow velocity in the direction of the conduit is further reduced by the presence of the divergent area formed by the narrowing of the cross-section of the channel which is passed through in the reverse direction. It is thus possible to achieve an at least temporary halt to the flow of the liquid metal in the supply conduit.

The invention also relates to a tip, a casting machine and a casting method which are characterised in combination by all or some of the features mentioned above or below.

Other aims, features and advantages of the invention will become apparent in view of the following description and the accompanying drawings, in which:

FIG. 1 shows a schematic, cross-sectional view of a casting machine in accordance with the invention,

FIG. 2 is schematic, cross-sectional view of tip in accordance with the invention,

FIG. 3 is a cross-sectional view of a tip in accordance with the invention during pinching of the flow of the metal.

FIG. 1 shows a longitudinal cross-sectional view of a casting machine 10 comprising a reservoir 11 suitable for containing liquid metal to be injected into a mould 30. The reservoir 11 can comprise heating means (not shown) to maintain the metal at its melting temperature, or be shaped into the form of a hopper in which the liquid metal is poured from a crucible. The liquid metal is then channelled in a conduit 12 for supplying the metal in the direction of a tip 20 fixed between the conduit 12 and the mould 30.

The conduit 12 is fitted with an electromagnetic pump 50 comprising a plurality of induction coils 51 regularly spaced along the conduit 12. Each of the induction coils 51 is connected to a power inverter 53 adapted to supply the coils 51 with alternating current. The induction coils 51 fulfil a dual role: on the one hand, they operate as induction heating coils permitting the stream of liquid metal flowing in the conduit 12 to be kept in the liquid state and, on the other hand, being supplied with polyphase alternating current adapted to the number and the order of the coils 51, they generate a moving magnetic field going from the reservoir 11 in the direction of the tip 20, this magnetic field making it possible to cause the liquid metal to flow in the conduit in the direction of the mould 30 at a velocity V0. By way of example, the induction coils are supplied with three-phase current at a voltage of 400V at a frequency of the order of 50 Hz to 10 kHz with a current variable between 50 A and 10 000 A.

The coils 51 also comprise a cooling circuit 52, e.g. using a cooling liquid circulating in copper tubes forming the coils 51, making it possible to limit heating thereof. Of course, the use of an air cooling system with forced convection employing one or more fans and cooling fins fixedly attached to the windings can also be contemplated.

Reference is made to FIG. 2 to detail the preferred structure of the tip 20. The tip 20 is fixed by means of flanges 25 to the end of the conduit 12 by its upstream end 23 (thus designated in relation to the direction of flow of the liquid metal in the conduit and in the tip). The tip 20 is also fixed to the mould 30 by flanges 25 at its opposite end, named downstream end 24. The tip 20 comprises a body 21 of refractory and electrically insulating material, preferably ceramic, and more particularly zirconium/aluminium nitride. Other refractory materials can also be used, e.g. alumina-based, zirconium-based, yttrium-based, titanium-based or nickel oxide-based ceramics, or even a mixture of these constituents in various proportions.

The body 21 of the tip is traversed by a channel 22 extending from the upstream end 23 to the downstream end 24 where the channel 22 issues into the injection opening 33 of the mould 30. The channel 22 is preferably of a revolutional cylindrical shape and comprises, at the upstream end 23, a conical part forming a narrowing 27 between a large cross-section with a diameter corresponding to the end of the conduit 12 and a smaller cross-section of the channel 22 corresponding to the cross-section of the injection opening 33 of the mould 30.

The body 21 also comprises, downstream of the narrowing 27, an electromagnetic coil 26 surrounding the channel 22 and over-moulded in the body 21. The coil 26 is preferably a multi-turn coil of copper or other highly conductive material, e.g. aluminium, cupro-beryllium, copper-chromium-zirconium alloy, tungsten or tungsten-copper alloy . . . . The coil 26 is suitable for connection to a current pulse generator 40 generally comprising a bank 41 of capacitors, which are charged by an external continuous supply source (not shown), and is discharged in the electromagnetic coil 26 via a spark gap 42. The coil 26 can also be formed by a single turn. Whether the coil 26 is a multi-turn or single-turn coil it has an axis of rotation substantially merged with at least a portion of the axis of the channel 21 over the part thereof which it surrounds. In the channel 21, the coil 26 thus defines a zone, named pinch zone 28, within which develops the electromagnetic field created by the coil 26 when this coil is supplied by the generator 40.

According to one variation which may be used when injection conditions permit, i.e. if the injection pressure and/or temperature and/or the metal to be injected are compatible with a tip having a body 21 with sufficiently thin walls, it is possible to place the coil 26 around the body 21. For example, for the injection of zinc alloys (without aluminium) having a melting point lower than 450° C., it is possible to use a tip of non-magnetic austenitic refractory stainless steel which permits satisfactory resistance with thicknesses reduced to a few millimetres. A single-turn or multi-turn electromagnetic coil 26 can thus be threaded onto the body 21 and fixed by any suitable means. This variation permits easy removal of the coil while retaining the body 21 of the tip.

Reference is now made to FIGS. 1 and 3 to describe the operation of the machine 10 and of the tip 20 as well as the casting method of the invention.

During a casting operation, the reservoir 11 is filled with a liquid metal, e.g. an alloy of zinc or magnesium. The liquid metal flows from the reservoir 11 into the conduit 12. The power inverter 53 supplies the induction coils 51 with a polyphase (e.g. three-phase) alternating current so as to induction heat the liquid metal in the conduit 12 to avoid any initiation of solidification or the formation of lumps. Each induction coil 51 also develops a magnetic field, the field lines of which are orientated along the axis of the induction coils and of the conduit 12. The phase difference in the magnetic field of the induction coils generates a moving magnetic field in the conduit 12 which displaces the liquid metal contained therein towards its end opposite to the reservoir 11 at a substantially constant velocity V0.

At the inlet of the tip 20, the flow cross-section of the liquid metal is reduced in the narrowing 27 and thus the velocity of displacement of the metal increases as a function of the cross-section ratio between the upstream end 23 of the tip and the cross-section of the channel 22 to reach a velocity V1 at the end of the narrowing, at the inlet into the pinch zone 28 located under the electromagnetic coil 26. The liquid metal progresses at the velocity V1 in the channel 22 then in the injection opening 33 of the mould 30 to fill one or more cavities 32 formed between the dies 31 of the mould 30. When the cavities 32 are filled, e.g. at the end of an injection time which is predetermined as a function of the velocity V1, the cross-section of the channel 22 and/or the injection opening 33 which define the metal throughput and the volume of the cavity or cavities 32, the pulse generator 40 is activated and delivers a current pulse, e.g. of an intensity of the order of 20 kA to 1 MA, a duration of 40 μs to 2 ms which flows in the coil 26. The large variation in the flux of the magnetic field generated by the coil 26 when this current pulse passes through it gives rise to a radial magnetic force which acts on the liquid metal 13 in the pinch zone 28. The liquid metal 13 is thus radially compressed and, owing to its incompressibility, is ejected axially on both sides of the pinch zone 28 at an ejection velocity V2. The combination of this ejection velocity V2 with the flow velocity V1 of the liquid metal in the direction of the mould 30 imparts to the metal a velocity corresponding to V1+V2. Taking into account that the cavity 32 is filled, this velocity V1+V2 is manifested as an increase in the injection pressure in the mould 30. By way of example, for a single-turn coil 26 of aluminium having a working zone (pinch zone 28) of a length of 15 mm, supplied by a current pulse Imax=500 kA to 500 Hz (2 ms), the coil generates a magnetic field of 40 T. When this magnetic field is applied to the liquid metal, this metal can achieve a maximum velocity (V1+V2) of the order of 30 m/s in the tip and a maximum pressure of 700 MPa.

On the other side of the pinch zone 28, the flow velocity of the metal following the combination of the flow and ejection velocities is V1-V2. In general, the ejection velocity V2 is higher in absolute value than the flow velocity V1 which imparts a movement to the liquid metal in the direction of the conduit 12 of the machine. By virtue of the narrowing 27 which behaves as a diverging area in this direction, the velocity of the liquid metal is decreased in the ratio of the cross-sections and does not generate a large shock wave in the conduit 12 likely to damage the machine, but simply a pressure wave which promotes the mixing of the metal in the conduit.

Of course, this description is given only by way of illustrative example and a person skilled in the art will be able to modify it in numerous ways without departing from the scope of the invention, e.g. by adapting the dimensions of the various elements of the casting machine according to the metal to be cast, the volume of the cavities of the mould etc. Similarly, the metering of the quantity of metal to be injected into the cavities of the mould can be regulated by an injection period prior to triggering the pulse generator or by regulating the velocity of transfer of the liquid metal by the frequency and phase of supply of the induction coils. 

1-10. (canceled)
 11. Tip (20) for injecting liquid metal (13) under pressure, said tip being tubular in shape, made from electrically insulating refractory material, suitable for being inserted between a conduit (12) for supplying liquid metal and an injection opening (33) of a mould (30), said tip comprising an injection channel (22) suitable for the flow of liquid metal between a first end, named upstream end (23), suitable for being connected to said conduit and a second end, named downstream end (24), suitable for being connected to the mould, wherein: said tip comprises an electromagnetic coil (26) placed between said ends, having an axis merged with at least one axis portion of the channel, suitable for being supplied with a current pulse from a high-voltage generator (40) and generating an electromagnetic pinch in the injection channel (22), the electromagnetic coil (26) is embedded in the material of the tip.
 12. Tip according to claim 11, wherein the injection channel (22) has a diameter comprising a narrowing (27) between the upstream end (23) and the electromagnetic coil (26).
 13. Tip according to claim 11, wherein the electromagnetic coil (26) is a multi-turn coil.
 14. Tip according to claim 11, wherein the electromagnetic coil (26) is a single-turn coil.
 15. Casting machine (10) comprising a liquid metal reservoir (11), a conduit (12) for the supply of liquid metal connected to said reservoir and fitted with an electromagnetic pump (50) suitable for causing the liquid metal to flow in said conduit in the direction of a mould (30), wherein the machine comprises a tip (20) according to claim 11 between said conduit and an injection opening (33) of said mould.
 16. Machine according to claim 15, wherein the electromagnetic pump (50) comprises a plurality of induction coils (51) which are coaxial to the conduit (12) for supplying liquid metal and which are suitable for induction heating of the metal flowing in said conduit.
 17. Machine according to claim 16, wherein the induction coils (51) are supplied with polyphase current so as to generate a moving magnetic field and to drive the liquid metal in the direction of the tip (20).
 18. Machine according to claim 16, wherein a device (52) for cooling the induction coils (50) is interposed between said coils and the conduit (12).
 19. Method for casting liquid metal under pressure in a metal mould (30), wherein: a casting machine (10) is used which comprises a liquid metal reservoir (11), a conduit (12) for supplying liquid metal, which is connected to said reservoir and fitted with an electromagnetic pump (50) suitable for causing the liquid metal to flow in said conduit in the direction of a mould (30), the electromagnetic pump (50) is supplied with a polyphase current so as to displace the liquid metal from the reservoir to the injection opening of the mould, wherein the casting machine (10) comprises a tip (20) comprising an electromagnetic coil (26) surrounding an injection channel (22) between said conduit and an injection opening (33) of said mould and in that, at the end of injection, the electromagnetic coil (26) of the tip (20) is supplied with an electric current pulse from a high-voltage generator (40) in order to generate an electromagnetic pinch in the injection channel (22) and to propel the liquid metal under pressure towards the injection opening (33) of the mould.
 20. Tip according to claim 12, wherein the electromagnetic coil (26) is a multi-turn coil.
 21. Tip according to claim 12, wherein the electromagnetic coil (26) is a single-turn coil.
 22. Casting machine (10) comprising a liquid metal reservoir (11), a conduit (12) for the supply of liquid metal connected to said reservoir and fitted with an electromagnetic pump (50) suitable for causing the liquid metal to flow in said conduit in the direction of a mould (30), wherein the machine comprises a tip (20) according to claim 12 between said conduit and an injection opening (33) of said mould.
 23. Casting machine (10) comprising a liquid metal reservoir (11), a conduit (12) for the supply of liquid metal connected to said reservoir and fitted with an electromagnetic pump (50) suitable for causing the liquid metal to flow in said conduit in the direction of a mould (30), wherein the machine comprises a tip (20) according to claim 13 between said conduit and an injection opening (33) of said mould.
 24. Casting machine (10) comprising a liquid metal reservoir (11), a conduit (12) for the supply of liquid metal connected to said reservoir and fitted with an electromagnetic pump (50) suitable for causing the liquid metal to flow in said conduit in the direction of a mould (30), wherein the machine comprises a tip (20) according to claim 14 between said conduit and an injection opening (33) of said mould.
 25. Casting machine (10) comprising a liquid metal reservoir (11), a conduit (12) for the supply of liquid metal connected to said reservoir and fitted with an electromagnetic pump (50) suitable for causing the liquid metal to flow in said conduit in the direction of a mould (30), wherein the machine comprises a tip (20) according to claim 15 between said conduit and an injection opening (33) of said mould.
 26. Casting machine (10) comprising a liquid metal reservoir (11), a conduit (12) for the supply of liquid metal connected to said reservoir and fitted with an electromagnetic pump (50) suitable for causing the liquid metal to flow in said conduit in the direction of a mould (30), wherein the machine comprises a tip (20) according to claim 16 between said conduit and an injection opening (33) of said mould.
 27. Machine according to claim 16, wherein a device (52) for cooling the induction coils (50) is interposed between said coils and the conduit (12). 